The subjects were babies, or as National Geographic photo editor Kurt Mutchler put it: "arguably the hardest portrait a photographer can attempt."

To keep our photographer on his toes, we decided that one cover baby wasn't enough. Why publish just one beautiful baby picture when you could do four? For the May story on the new science of longevity, four different covers are being distributed to subscribers and newsstands. Alongside a headline reading "This baby will live to be 120," one of four cherubic faces serenely gazes at the reader.

Our editors had so much fun with the cover that the magazine created a Facebook app so you can put your own baby pictures within our iconic yellow border. See how your favorite baby photo will look at natgeobabycover.com.

Photographing babies is a challenge for any photographer. "Kids are complicated and different," said photographer Robert Clark, a National Geographic veteran with more than a dozen covers to his name. "Sometimes it was super, super easy, and other times it was incredibly difficult."

Clark's assignments have had him photographing Machu Picchu at night and taking extreme close-ups of a gecko's toes, but his only experience photographing babies was personal: his daughter, Lola, and a nephew.

To get the final four cover images Clark photographed more than 20 babies. He asked friends, called local playgroups, posted fliers in his neighborhood, and even tried modeling agencies for "professional babies." The assignment lasted four days. He kept the lighting in the studio exactly the same and never moved his camera from its perch on some scaffolding, all to make sure that the only variable was the child in front of the lens.

Each baby was in front of the camera for about 10 minutes, although for some "after two minutes, you knew it wasn't going to work," Clark said. Parents were on hand to pull funny faces and make noises, or to whisk their kids away if the shoot became too much for them. "We have almost every kid laughing their head off, and I think every single child cried at some point," Clark said. The children were between the ages of 6 months and 18 months; many had mastered the art of walking and were more interested in exploring the studio than staring at a lens.

Clark had one secret weapon from his now four-year-old daughter's younger years: "a funny little toy called a Sophie" —a rubber giraffe popular in France. "Practically every kid responded to Sophie," he said.

For parents hoping to photograph their own children, Clark's advice is to "shoot a lot" and get to know their cell phone cameras. "The most important camera is the one you have, and people always have their cell phones on them," Clark said. "You might as well use it the best you can."